Glancing incidence telescopes such as the Wolter x-ray telescopes are typically used to focus the x-rays from a point source (or an extended source) at infinity to a high resolution image on the sensitive surface of the detector situated at the prime focus of the Wolter telescope. For soft x-rays (wavelengths ranging from 2.ANG. to 100.ANG.), the Wolter type I mirror system with concave paraboloidal and hyperboloidal elements (all of which are coaxial, confocal and internally reflecting) is typically used. Such telescopes were flown on the Skylab space station and have been used on the Einstein and Copernicus observatories in space. For very soft x-rays and extreme ultraviolet (XUV) radiation (100.ANG. to 600.ANG.) range Wolter type II systems are typically used. These differ from the Wolter I configuration by virtue of the fact that the second reflecting element is a convex, externally reflecting hyperboloid usually mounted partially within the confines of the paraboloidal mirror. In some cases, to improve off-axis performance, the exact contours of these elements are modified in accordance with the Wolter-Schwarzschild configuration. A number of these systems have been built and flown on sounding rockets, and on the Apollo spacecraft.
Historically, the spatial resolution of glancing incidence x-ray telescope systems has been limited by the detector used rather than the x-ray optics. High spatial resolution x-ray detectors (such as photographic film) tend to be of low quantum efficiency; whereas high quantum efficiency detectors tend to have intrinsically low spatial resolution characteristics. Hence to use a device such as a charged coupled device (CCD), which is extremely sensitive over a very broad wavelength range, it is necessary to have an x-ray telescope of very great focal length to achieve a plate scale that allows high resolution imagery with the CCD. Very long telescopes are typically heavy, and in space applications they pose significant mobility constraints upon the launch vehicle, instrument pointing system, alignment tolerances, and thermal control system.
Alteration of the telescope plate scale can be achieved by coupling the Wolter mirror system to the detector by a relay optic system such as a glancing incidence hyperboloid/ellipsoid x-ray microscope optic. The resultant system still has a longer physical length than the focal length of the Wolter mirror system, but the length of this system is very much less than that which would be required if one simply designed the Wolter optic to provide the equivalent plate scale. The primary disadvantage of this approach is that the x-ray microscope optics equipment is extremely expensive. Furthermore, the alignment tolerances are tight and the system must be provided with appropriate thermal control to insure that the microscope optic remains with its front focal plane accurately positioned on the primary focal plane of the Wolter mirror system. Also, the microscope provides no spectral discrimination, although this would be considered an advantage if the microscope optic were used to feed a high-spectral-resolution crystal spectrometer.
The approach of using the x-ray microscope optic to give the system a long effective focal length has one further disadvantage when compared with the present invention. Once the microscope magnification has been chosen, it becomes fixed, thus rigidly fixing the resultant field of view. The mirrors of the present invention are sufficiently simple to build and inexpensive that many can be used, each of which provides a different effective focal length, field of view, and the same or different spectral slice. The spectral slicing x-ray telescope can be used somewhat like a zoom lens, with the magnification, spectral slice, and field of view altered simultaneously by simply positioning a different hyperboloidal mirror into the converging x-ray beam.
A significant disadvantage of the prior art lies in the use of thin foils as filter materials for obtaining spectral information. The great overlap in the spectral response of various filters as well as their wide bandpass make these filters have very limited value. Also, these devices are of virtually no value in the very soft x-ray/XUV region since to transmit the long wavelengths they must be very thin, making them pass harder radiation as well. There is great difficulty in fabricating very thin window filters, and they must be suspended upon some type of support mesh. These filters are also very prone to failure due to their inherently low structural strength. Recently, however, this situation has been somewhat improved by fabrication of filters composed of multiple layers of several different elements.